479 



Lake — where flood factors are largely excluded — and the ther- 

 mograph (air) for 1896 will serve to suggest the possibility of a 

 causal nexus between the two phenomena of fluctuations in 

 heat and some of the movements in plankton production. The 

 many exceptions to any close correlation emphasize, however, 

 the fact that heat is only one of the many factors involved in 

 the problem, and also indicate the necessity for much fuller 

 plankton data, with closer interval and the proper quantitative 

 representation of the minute forms now lost by leakage through 

 the silk, for any adequate discussion of the problem. The 

 present data serve only to suggest the problem for investiga- 

 tion. 



The effect of the ice-sheet 'upon the course of plankton pro- 

 duction is apparent in a number of instances in our records. 

 The most noticeable case was the extermination of the plank- 

 ton in the channel in February, 1895, by the ice-sheet of two 

 months' duration; but this catastrophe was not repeated else- 

 where in our records in this or other years. Indeed, owing to 

 the fact that the period of the ice blockade is usually one of 

 lower levels and more stable conditions, we find generally that 

 production under the ice, even at minimum temperatures, 

 rises above prior or subsequent levels. An inspection of the 

 plates, especially those of 1898, will show repeated instances of 

 this phenomenon in both channel and backwaters. One of the 

 most striking phenomena in all our records is this winter pro- 

 duction under the ice-sheet in 1898-1899, a production which 

 in the river attained an amplitude in December (.99) not 

 equaled since June, and in February (.81) one surpassed only 

 by the August (.91) and December means. In Quiver Lake 

 likewise, the December (1.74), January (.77), and February 

 (1.05) means are all considerably in excess of the June-Novem- 

 ber production, the average of the winter months (1.19) being 

 over threefold greater than that of the warmer months (.33) 

 named. In Thompson's Lake also the midwinter production 

 in this season was large, reaching an average of 1.94 for the 

 winter months above named, and only 1.96 for the five preceding 



